Expression of Fgfr2 transcripts in embryonic gonads. (A–C) mRNA in situ hybridization of Fgfr2 and Sox9 on frozen sections of XX and XY gonads at 12.5 dpc. Fgfr2 antisense riboprobe reveals Fgfr2 expression in the coelomic domain (arrowhead) of XX and XY gonads and inside the testis cords (arrow), but not in the interstitium, of XY gonads. Sox9 is a comparative control for expression in Sertoli cells inside testis cords. (D) RT-PCR analysis of splicing isoforms of Fgfr2 in fluourescence activation-sorted Sertoli cells. A confocal image of Sox9-Ecfp XY gonad shows Sertoli cells marked with EGFP (arrowheads). EGFP+ (Sertoli) cells express mainly Fgfr2-IIIc; whereas EGFP− cells express both isoforms. PCR products are Hprt (white arrowhead), Fgfr2-IIIb (red arrowhead), Fgfr2-IIIc (green arrowhead), and the tyrosine kinase domain of Fgfr2 (black arrowhead).